Fluid catalytic cracking catalysts generally contain a framework structure comprising a crystalline tetrahedral framework oxide component, an active porous inorganic oxide catalyst framework component, and an inert catalyst framework component. The framework structure is typically held together by attachment with an inorganic oxide matrix component.
Each component of a fluid catalytic cracking catalyst has its own particular function. The tetrahedral framework oxide component catalyzes the breakdown of primary products from the catalytic cracking reaction into clean products such as naphtha for fuels and olefins for chemical feedstocks. The active porous inorganic oxide catalyst framework component catalyzes the formation of primary products by cracking hydrocarbon molecules that are too large to fit inside the tetrahedral framework oxide component. The inert catalyst framework component densifies, strengthens and acts as a protective thermal sink. The inorganic oxide matrix binds the catalyst framework structure components together so that the catalyst product which is formed is hard enough to survive interparticle and reactor wall collisions.
In addition to acting as a binder material, the matrix component also serves as a reactive medium for the diffusion of feedstock and cracked products. In general, the matrix will have a pore structure that allows the diffusion of hydrocarbon molecules in and out of the catalyst particles. This pore structure is desirably one that will not deteriorate during severe hydrothermal treatment of the catalyst. By favoring or inhibiting the diffusion of certain hydrocarbon molecules, the pore structure will affect the activity and selectivity of the catalyst.
The matrix also serves as a diluting medium for the crystalline tetrahedral framework oxide component. This moderates catalyst activity and avoids overcracking of the products to coke and gas.
The matrix can also act as a sink for sodium ions. Through solid-solid ion exchange, the sodium ions migrate from the crystalline tetrahedral framework oxides into the matrix, which increases the thermal and hydrothermal stability of the catalyst.
The matrix further acts as a heat carrier for typical fluidic catalytic cracking systems. By facilitating heat transfer during both the cracking and regeneration steps, the crystalline tetrahedral framework oxide is provided additional protection from structural damage.
Until recently, the crystalline tetrahedral framework oxide content of catalytic cracking catalysts was low enough such that the structure of the matrix was tailored to favor activity and selectivity over strength (i.e., attrition resistance). However, present catalytic cracking catalysts typically contain a relatively high concentration of crystalline tetrahedral framework oxide; as much as 60 wt %. At relatively high crystalline tetrahedral framework oxide concentrations, the matrix component must be manufactured to have increased attrition resistance, while maintaining activity and selectivity.
Matrices of catalytic cracking catalysts have historically been formed from simple amorphous gels of silica-alumina or silica-magnesia. These gels contained agglomerated sol particles having pore diameters, on drying, in the range from 20 .ANG. to 120 .ANG.. Matrices based on silica and alumina sols have also been developed.
Catalysts manufactured with sols do not have particularly desirable pore structure, although these catalysts typically have relatively good attrition resistance. The undesirable pore structure is primarily due to the fact that the sol particles are generally so small that they can "blind" the pores of the zeolite. The result in most cases is that the pore structure is too small to effectively crack large gas oil molecules into intermediate products so that the intermediate products can enter into and be cracked inside the crystalline tetrahedral framework oxide component.
In an effort to balance pore structure with attrition resistance, monodispersed mesoporosity has been introduced into FCC catalysts. "Monodispersed mesoporosity" in this context means that a substantial portion of the pore structure above 150 .ANG. is provided by the interaction of components whose ultimate particle size is approximately one-half to one-third the desired pore diameter. Materials made in this way tend to have a well-defined region of mesoporous behavior. This type of pore structure reflects an underlying relationship between the catalyst components which confer greater strength and catalyst components which provide accessibility to the catalytically active components.
Catalysts which have a pore structure above about 150 .ANG. do not typically have enough surface area to efficiently convert large gas oil molecules to the distillate range fraction which can enter into the crystalline tetrahedral framework oxide. Therefore, it is useful to employ material s which themselves have surface areas in the 30-150 .ANG. pore diameter region or which can be converted into material s with surface areas in the 30-150 .ANG. pore diameter region. However, many of the prior art catalysts having pore diameters in the 30-150 .ANG. range have undesirably high coke yields. See, for example, EP 350 280 which discloses catalyst greater than about 90 .ANG. in diameter as being more desirable compared to smaller diameter catalyst such as that described in U.S. Pat. No. 3,944,482.